Hybrid intrastromal corneal ring

ABSTRACT

This invention is an intrastromal corneal ring having comprising at least one outer layer of a physiologically compatible polymer having a low modulus of elasticity, which polymer may be hydratable and may be hydrophilic. The inner portion of the hybrid intrastromal corneal ring may be hollow or may contain one or more physiologically compatible polymers.

This application is a division of application Ser. No. 07/927,165, filedAug. 7, 1992, now abandoned.

FIELD OF THE INVENTION

This invention is a hybrid intrastromal corneal ring (“ICR”) comprisingat least one outer layer of a physiologically compatible polymer havinga low modulus of elasticity, which polymer may be hydratable and may behydrophilic. The inner portion of the ICR may be hollow or may containone or more physiologically compatible polymers.

BACKGROUND OF THE INVENTION

Anomalies in the overall shape of the eye can cause visual disorders.Hyperopia (“farsightedness”) occurs when the front-to-back distance inthe eyeball is too short. In such a case, parallel rays originatinggreater than 20 feet from the eye focus behind the retina. In contrast,when the front-to-back distance of eyeball is too long, myopia(“nearsightedness”) occurs and the focus of parallel rays entering theeye occurs in front of the retina. Astigmatism is a condition whichoccurs when the parallel rays of light do not focus to a single pointwithin the eye, but rather have a variable focus due to the fact thatthe cornea is aspherical and refracts light in a different meridian atdifferent distances. Some degree of astigmatism is normal, but where itis pronounced, the astigmatism must be corrected.

Hyperopia, myopia, and astigmatism are usually corrected by glasses orcontact lenses. Surgical methods for the correction of such disordersare known. Such methods include radial keratotomy (see, e.g., U.S. Pat.Nos. 4,815,463 and 4,688,570) and laser corneal ablation (see, e.g.,U.S. Pat. No. 4,941,093).

Another method for correcting those disorders is through theimplantation of polymeric rings in the eye's corneal stroma to changethe curvature of the cornea. Previous work involving the implantation ofpolymethylmethacrylate (PMMA) rings, allograft corneal tissue, andhydrogels is well documented. One of the ring devices involves a splitring design which is inserted into a channel previously dissected in thestromal layer of the cornea. A minimally invasive incision is used bothfor producing the channel and for implanting the implant.

The use of instastromal corneal rings made completely of certain hard,hydrophobic polymers is known. For instance, in the article“Intrastromal Implantation Eines Justierbaren Kunststofforings ZurHornhautrefraktionsanderung”, Hartmann et al., Kongress der DeutschenGesellschaft fur Intraokularingen Implantation, delivered by h. Fryleret al., Spring-Verlag, Wien, pages 465-475, the use of intrastromal ringimplants of rings of silicone, polymethylmethacrylate (“PMMA”), andfluorocarbons TEFLON®, a PTFE available from DuPont. Other disclosuresof the use of PMMA in such intrastromal rings is found in U.S. Pat. No.4,452,235 to Reynolds; U.S. Pat. No. 4,671,276 to Reynolds; U.S. Pat.No. 4,766,895 to Reynolds; and U.S. Pat. No. 4,961,744 to Kilmer et al.These documents do not suggest the use of multiple-layers of differingmaterials in the intrastromal corneal ring.

The use of soft polymers as intrastromal inserts is not widely known.For instance, U.S. Pat. No. 5,090,955 to Simon, suggests an ICR which ismade by introducing a settable polymer into an intrastromal channelwhich has been previously made and allowing the polymer to set. Thisprocedure does not allow the surgeon to specify the size of theresulting ring nor is it a process which allows control of the flowingpolymer within the eye.

Temirov et al, “Refractive circular tunnel keroplasty in the correctionof high myopia”, Vestnik Oftalmologii 1991: 3-21-31, suggests the use ofcollagen thread as intrastromal corneal rings material.

They specifically do not suggest the use of soft or hydrophilic polymersinsertable into the cornea as intrastromal corneal rings.

SUMMARY OF THE INVENTION

This invention is a hybrid intrastromal corneal ring comprising at leastone outer layer of a soft, low modulus, often hydrophilic,physiologically compatible polymer.

The portion of the ICR inside that outer layer may be hollow and adaptedto be fillable with a biologic agent, drug or other liquid, emulsified,or time-release eye treatment material. The inner portion may comprisevariously a core of a high modulus, physiologically compatible polymeror a further composite of a low modulus polymer or a high moduluspolymer core or a high modulus polymer or a low modulus polymer core.The inner portion may comprise a polymeric material which is polymerizedin situ after introduction into a hollow center layer.

The term “high modulus polymer” is meant to include physiologicallycompatible polymers such as PMMA; TEFLON; certain longer chainsilicones; polycarbonate; and polyolefins such as polyethylene,polypropylene, polybutylene, their mixtures or other polyolefininterpolymers. The term “low modulus polymer” is meant to includephysiologically compatible polymers and hydrogels, such aspolyhydroxyethyl methacrylate (“polyHEMA”) or polyvinylpyrrolidone(“PVP”) or elastomeric materials and biologic polymers such ascrosslinked dextran, hyaluronic acid, and heparin or the like. The lowmodulus hydratable polymers, in any case, may be of the type which issufficiently cross-linked such that they do not swell upon contact withwater (and subsequent hydration) or of the type which swells whenhydrated. Additionally, the class of low modulus polymers is meant toinclude elastomeric polymers, e.g., latex rubber, colloids of polyesterand polyether, polyurethanes, lower molecular weight silicones,isoprene, and the like, which are stable and physiologically compatible.Finally, the low modulus polymer may be a reinforced hydrogel such as aninterpenetrating network of polymerized vinyl pyrrolidone and methylmethacrylate.

Our intrastromal corneal rings may be implanted into the stroma using anumber of known techniques. If hydratable polymers are used, they may behydrated before or after introduction into the intrastromal passagewaycreated by the surgical device used to introduce these devices into theeye. If the outer layer is hydrated before insertion into the eye, thefinal size of the ring is set before that insertion. If the hydratablepolymers are allowed to hydrate within the corneal space, the device (ifappropriate polymers are chosen) will swell within the eye to its finalsize. If prehydrated, the outer layer often provides a measure oflubricity to the ICR, allowing it to be inserted with greater ease.Other of the noted low modulus polymers may also provide such lubricity.

This invention allows for adjustment of intrastromal corneal ringsthickness and diameter and provides a softer interface between a harderpolymer core and corneal tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a horizontal section of the eye.

FIG. 2 is a schematic illustration of the anterior portion of the eyeshowing the various layers of the cornea.

FIG. 3 shows the step of inserting the inventive intrastromal cornealring through an incision in the cornea.

FIG. 4 shows, in partial cross-section, the anterior portion of an eyewith the hybrid intrastromal corneal ring installed.

FIGS. 5A and 5B show, in cross-section, respectively, an unhydrated anda hydrated ring in which the hydratable, low modulus polymer is placedon only two surfaces.

FIG. 6 shows in cross-section an unhydrated hybrid ICR in which thehydratable, low modulus polymer completely coats the ring.

FIG. 7 shows a multiply-coated intrastromal corneal ring.

FIG. 8 shows, in cross-section, an intrastromal corneal ring comprisinga swellable, low modulus polymer.

FIGS. 9A and 9B show, in cross-section, an ICR comprising a fillableshell which is nonhydrated in FIG. 9A and hydrated and swollen in FIG.9B. The FIG. 9B intrastromal corneal ring contains a fluid.

FIGS. 10A and 10B show a baffled soft intrastromal corneal ring.

FIG. 11 shows an end-to-end intrastromal corneal ring connector whichpermits introduction of drugs or a settable polymer.

DESCRIPTION OF THE INVENTION

Prior to explaining the details of the inventive devices, a shortexplanation of the physiology of the eye is needed to appreciate thefunctional relationship of the intrastromal corneal ring to the eye.

FIG. 1 shows a horizontal cross-section of the eye with the globe (10)of the eye resembling a sphere with an anterior bulged spherical portionrepresenting the cornea (11).

The globe (10) of the eye consists of three concentric coveringsenclosing the various transparent media through which the light mustpass before reaching the light-sensitive retina (12). The outermostcovering is a fibrous protective portion the posterior five-sixths ofwhich is white and opaque and called the sclera (13), and sometimesreferred to as the white of the eye where visible to the front. Theanterior one-sixth of this outer layer is the transparent cornea (11).

A middle covering is mainly vascular and nutritive in function and ismade up of the choroid (14), ciliary body (15), and iris (16). Thechoroid generally functions to maintain the retina (12). The ciliarybody (15) is involved in suspending the lens (17) and accommodation ofthe lens. The iris (16) is the most anterior portion of the middlecovering of the eye and is arranged in a frontal plane. It is a thincircular disc similar in function to the diaphragm of a camera, and isperforate near its center by a circular aperture called the pupil (20).The size of the pupil varies to regulate the amount of light whichreaches the retina (12). It contracts also to accommodation, whichserves to sharpen the focus by diminishing spherical aberration. Theiris divides the space between the cornea (11) and the lens (17) into ananterior chamber (21) and posterior chamber (22). The innermost portionof covering is the retina (12), consisting of nerve elements which formthe true receptive portion for visual impressions.

The retina (12) is a part of the brain arising as an outgrowth from thefore-brain, with the optic nerve (23) serving as a fiber tractconnecting the retina part of the brain with the fore-brain. A layer ofrods and cones, lying just beneath a pigmented epithelium on theanterior wall of the retina serve as visual cells or photoreceptorswhich transform physical energy (light) into nerve impulses.

The vitreous body (24) is a transparent gelatinous mass which fills theposterior four-fifths of the globe (10). At its sides it supports theciliary body (15) and the retina (12). A frontal saucer-shapeddepression houses the lens.

The lens (17) of the eye is a transparent bi-convex body of crystallineappearance placed between the iris (16) and vitreous body (24). Itsaxial diameter varies markedly with accommodation. A ciliary zonule(25), consisting of transparent fibers passing between the ciliary body(15) and lens (17) serves to hold the lens (17) in position and enablesthe ciliary muscle to act on it.

Referring again to the cornea (11), this outermost fibrous transparentcoating resembles a watch glass. Its curvature is somewhat greater thanthe rest of the globe and is ideally spherical in nature. However, oftenit is more curved in one meridian than another giving rise toastigmatism. A central third of the cornea is called the optical zonewith a slight flattening taking place outwardly thereof as the corneathickens towards its periphery. Most of the refraction of the eye takesplace through the cornea.

FIG. 2 is a more detailed drawing of the anterior portion of the globeshowing the various layers of the cornea (11) making up the epithelium(31) and the general placement of the ICR (40). Epithelial cells on thesurface thereof function to maintain transparency of the cornea (11).These epithelial cells are rich in glycogen, enzymes and acetylcholineand their activity regulates the corneal corpuscles and controls thetransport of water and electrolytes through the lamellae of the stroma(32) of the cornea (11).

An anterior limiting lamella (33), referred to as Bowman's membrane orlayer, is positioned between the epithelium (31) and the stroma (32) ofthe cornea. The stroma (32) is made up of lamellae having bands offibrils parallel to each other and crossing the whole of the cornea.While most of the fibrous bands are parallel to the surface, some areoblique, especially anteriorly. A posterior limiting lamina (34) isreferred to as Descemet's membrane. It is a strong membrane sharplydefined from the stroma (32) and resistant to pathological processes ofthe cornea.

The endothelium (35) is the most posterior layer of the cornea andconsists of a single layer of cells. The limbus (37) is the transitionzone between the conjunctiva and sclera on the one hand and thecornea(11) on the other.

FIG. 3 shows the completion of the step of inserting the hybridintrastromal corneal ring into the corneal stroma. Techniques such asthat shown in our copending application Ser. No. 07/867,745, entitledCORNEAL VACUUM CENTERING GUIDE AND DISSECTOR, filed Apr. 10, 1992, aresuitable for preparing the eye and inserting the intrastromal cornealring into the appropriately prepared interlamellar stromal channel.Generally the ICR (50) is installed in the following manner: A smallradial incision (52) is made at the corneal radius in which theintrastromal corneal ring is ultimately to be installed about thecornea. A dissector in the form of a split ring having a point suitablefor producing the interlamellar channel in the corneal stroma isintroduced into the stromal space through the small incision. It is thenrotated in such a fashion that a generally circular channel is formedcompletely encircling the cornea. The dissector is then rotated in theopposite direction to withdraw it from the tunnel or channel thusformed. An intrastromal corneal ring is then introduced into thecircular channel and joined at its ends. Hybrid intrastromal cornealring having partially hydrated polymers on their outer periphery aretypically slippery and consequently may be introduced into theinterlamellar tunnel with great ease. It is usually desirable to (atleast partially) hydrate the hybrid ICR in that, otherwise, the ICRduring its traverse through the tunnel may desiccate the path and beginto stick to the interior wall of the tunnel. The ICR may be lubricatedwith suitable ocular lubricants such as hyaluronic acid, methylethylcellulose, dextran solutions, glycerine solutions, polysaccharides, oroligosaccharides upon its introduction to help with the insertionparticularly if one wishes to insert the hybrid intrastromal cornealring without any hydration.

FIG. 4 shows, in cross-section, the anterior portion of the eye with thehybrid intrastromal corneal ring (50) inserted. Subsequent to theinsertion, the intrastromal corneal ring (50) will swell to its finalsize or thickness (53) within the eye. This swelling permits theinclusion of larger intrastromal corneal ring than would normally beaccommodated within normal sized intrastromal channels.

FIG. 5A shows in cross-section the hybrid intrastromal corneal ringhaving inner and outer faces (54) comprising polymers having low moduliof elasticity. Low modulus polymers are those having a modulus ofelasticity below about 3.5 kpsi, more preferably between 1 psi and 1kpsi, and most preferably between 1 psi and 500 psi. They must bephysiologically compatible with the eye. Most polymeric materials usedin soft contact lenses are suitable for the outer layer contemplated inthis invention. In addition, the class includes physiologicallycompatible elastomers such a polyacrylates, silicones, isoprene, and thelike. Additionally, low modulus polymers include biologic polymers suchas crosslinked dextran, crosslinked heparin or hyaluronic acid.

The inner portion or core (56) as shown in FIG. 5A is a physiologicallycompatible polymer having a high modulus of elasticity. A high modulusof elasticity is considered to be greater in value than about 3.5 kpsi,preferably 5-12 kpsi, and most preferably 8-10 kpsi. These polymers aretypically stiffer and may be materials such as polymethyl methacrylate(PMMA), TEFLON (PTFE), longer chain silicone polymers such as are usedin hard contact lenses. Additionally, suitable polymers includepolycarbonates; polyolefins such as polyethylene, polypropylene, andpolybutylene, their mixtures and polyolefin interpolymers, blockcopolymers and the like.

The extent to which the outer layers swell upon hydration is dependentupon the type of polymer chosen and, when the polymer is hydratable,upon the amount of cross-linking found in the outer layers (54), and onthe thickness of the layer. Generally speaking, the more highly linkedthe hydratable polymer, the smaller the amount of volume change uponhydration. Conversely, a polymer having only sufficient cross-linkingfor strength in the service in which this device is placed, will have asomewhat lower level of cross-linking. Alternatively, a substantiallynonswellable polymer system may be formed of a hydrogel physicallyinterpenetrated by another polymer which does not hydrate, e.g.,polyHEMA with polyacylnitrite.

The thickness of the outer layer depends in large function upon theintended use of the ICR. For instance if the outer layer is to be usedas a container for an inner volume of a settable polymer or drug, theouter layer may be relatively thicker. If the outer layer is used toprovide a swellable outer layer which does not add significantly to thesize of the ICR or is used functionally as a lubricant layer, the otherlayer may be quite thin even to the point of a layer of minimumcoverage, perhaps as thin as a molecule thick.

FIG. 5B shows the hybrid ICR of FIG. 5A after it has been completelyhydrated and the polymer faces (54) have swollen to their largestextent.

The inventive device shown in FIGS. 5A and 5B may also be used in theinstance where a low modulus covering is not placed over the entireoutside surface of the intrastromal corneal ring. For instance, toalleviate astigmatism, an ICR having a thick portion and a thin portionmay be desired. An ICR having an inner core of a high modulus polymerand an outer covering of a swellable polymer might be chosen. Thesurgeon would remove a portion of the intrastromal corneal ring exteriorcoating or face prior to introducing the ICR into the eye. Such anintrastromal corneal ring and its use are described more fully in Ser.No. 07/939,492, entitled ASTIGMATIC CORRECTING INTRASTROMAL CORNEALRING, filed Sep. 3, 1992.

FIG. 6 shows a hybrid intrastromal corneal ring in which the core (56)is of high modulus polymeric material such as mentioned above. In thisvariation the outer surface is completely coated with a swellablepolymer or polymeric gel such as those discussed above. Again, thecomposition of outer covering (58) may be of a hydratable polymer systemwhich is sufficiently cross-linked that the polymer does not swellappreciably upon hydration. Alternatively, the covering (56) may becross-linked only so much as to allow substantial swelling of the outercovering either before or after insertion into the eye.

FIG. 7 shows another variation of the inventive hybrid intrastromalcorneal ring. In this variant of the inventive hybrid intrastromalcorneal ring, the inner high modulus core (56) is surrounded by morethan one layer, specifically, an intermediate layer (60) and an outerlayer (62). This hybrid intrastromal corneal ring may be appropriatewhen the outer layer (62) is difficult to bond to the core (56). Anintermediate layer (60) which bonds both to the core (56) and to theouter layer (62) may be used. Intermediate layer (60) typically wouldnot be a polymer which swells appreciably upon hydration lest it splitouter layer (62). Outer layer (62) may either swell upon hydration, asis shown in FIG. 7, or may remain at approximately the same size uponhydration if a suitably low modulus polymer system is chosen.

FIG. 8 shows an intrastromal corneal rings made of a low modulus,hydratable polymer such as those discussed above. Since these polymersoften lose substantial mechanical strength upon hydration, these ICRswould be inserted into the intrastromal space prior to being hydrated orwith the assistance of a tool either before or after hydration.

FIG. 9A shows an intrastromal corneal ring made of a low modulus polymersystem hydratable outer coating (66) and an inner cavity (68). Thisintrastromal corneal ring may be inserted into the intrastromal spacecreated by the dissector (as described above) as a covering on a toolsimilar to the dissector which created the intracorneal channel. Once inposition the insertion tool is rotated out of the intrastromal cornealring leaving the shell within the stroma.

Alternatively, the intrastromal corneal ring may be introduced into theintrastromal channel as the dissector is fully rotated in place,attached to the leading edge of the dissector, and pulled into theintrastromal channel as the dissector is rotated out of the eye.

FIG. 9B shows the intrastromal corneal ring of FIG. 9A upon completionof the hydration in that the outer covering (66) is swollen to itslargest extent. Furthermore, the inner cavity (68) (FIG. 9A) may befilled with a biologic, drug or other liquid, or biologically active eyetreatment material (7 a). These devices may be tied or otherwiseconnected at their point of insertion by known techniques.

The shell may be injected with a settable soft polymer core and allowedto expand to a desired thickness. Suitable injectable polymers are wellknown but include polyHEMA hydrogel, cross-linked collagen, cross-linkedhyaluronic acid, siloxane gels, and organic-siloxane gels such ascross-linked methyl vinyl siloxane gels. The injected polymer sets afterinjection.

FIGS. 10A and 10B show a variation of the inventive intrastromal cornealring (80) in which a polymeric ring containing a number of chambers (82)separated by baffle walls (84) which may have an optional hole (86) inthe baffle wall (84). Once the intrastromal corneal ring is introducedinto the eye, it may be filled with a drug or biologic material viainjection with a suitable syringe through the intrastromal corneal ringwall or into the end (88) of the tube. A settable polymer of the typediscussed above may be introduced into the chambers in similar fashionif a ring of variable bulk is required. It is also appropriate toinclude a smaller fill tube (90) which extends through the end (88) ofthe tube (80) through the holes (86) in the baffle walls (84). In thisway drugs, biologic agents, or settable polymers may be delivered in aspecific fashion throughout the ring as the fill tube is withdrawn fromthe intrastromal corneal ring.

FIG. 11 shows a connector (92) situated between the ends of anintrastromal corneal ring (96). The intrastromal corneal ring is a lowmodulus polymer having a hollow center. The connector has a receiverport (94) suitable for introducing drugs, biologics, or settablepolymers into the interior of the intrastromal corneal ring. Theconnector has a passageway connecting the receiver port (94) into theintrastromal corneal ring interior.

In the variation of the invention specified just above and shown inFIGS. 10A, 10B, and 11, the wall or outer covering of the intrastromalcorneal ring may be a hydratable low modulus polymer or an elastomer. Ifan elastomeric polymer is selected, the intrastromal corneal ring may beinjected with a settable polymer as specified.

The low modulus polymers used in this invention are often absorbent,particularly if they are hydratable, and may be infused with a drug orbiologic agent which may be slowly released from the device afterimplantation of the intrastromal corneal ring. For instance, the lowmodulus polymer may be loaded with a drug such as dexamethasone toreduce acute inflammatory response to implanting the device. This drughelps to prevent undesirable scarring or vascular ingrowth toward theintrastromal corneal ring. Similarly, heparin, corticosteroids,antimitotics, antiinflammatories, and antiangiogenesis factors (such asnicotine adenine dinucleotide (NAD⁺)) may be included to reduce orprevent angiogenesis and inflammation.

Clearly, there are a variety of other drugs suitable for inclusion inthe intrastromal corneal ring. The choice will depend upon the use towhich the drugs are put.

The terms and expressions which have been used in the description aboveare used only as terms of description and not of limitation. There is nointention of excluding equivalents of the features shown or described.It is recognized that one having ordinary skill in this art wouldperceive equivalence to the inventions claimed below, which equivalencewould be within the spirit of the invention as expressed above.

I claim as my invention:
 1. An intracorneal ring adapted forimplantation within a human cornea and configured to alter the curvatureof the cornea to effect refractive correction of an eye, said ringcomprising at least one inner layer comprising a high elastic modulus,physiologically compatible polymer and at least one outer layercomprising a low elastic modulus, physiologically compatible polymer. 2.The ring of claim 1 wherein the low elastic modulus physiologicallycompatible polymer is selected from hydratable polymers which swell uponhydration and hydratable polymers which do not swell upon hydration. 3.The ring of claim 2 wherein the low elastic modulus polymer comprises apolymer selected from polyhydroxyethyl methylacrylate andpolyvinylpyrrolidone.
 4. The ring of claim 1 wherein the low elasticmodulus, physiologically compatible polymer comprises an elastomer. 5.The ring of claim 1 wherein said high elastic modulus polymer comprisesat least one polymer selected from PMMA, poly(tetrafluoroethylene),silicone, polycarbonate, a polyolefin selected from polyethylene,polypropylene, polybutylene, and mixtures or interpolymers thereof. 6.The ring of claim 1 wherein the low elastic modulus polymer comprises atleast one polymer selected from latex rubber, polysilicones,polyurethanes, polyesters, and polyacrylates.
 7. The ring of claim 1wherein said ring comprises a third layer between the outer layer andthe inner layer having an adhesion to the inner layer and to the outerlayer that is greater than the adhesion of the inner layer to the outerlayer.
 8. The ring of claim 1 wherein said ring is a split ring.
 9. Anintracorneal ring adapted for implantation within a human cornea andconfigured to alter the curvature of the cornea to effect refractivecorrection of an eye, said ring comprising a physiologically compatiblepolymeric inner portion and a physiologically compatible polymeric outerportion, said outer portion having a low elastic modulus and said innerportion having an elastic modulus that is greater than the elasticmodulus of the outer portion.
 10. The ring of claim 9 wherein the innerportion has a high elastic modulus.
 11. The ring of claim 9 wherein thering is a split ring.
 12. An intracorneal implant shaped to be insertedin an intracorneal channel in a circumferential direction about theoptic axis of a human eye and configured to alter the curvature of thecornea to effect refractive correction of an eye, said implantcomprising an elongated arcuate member having ends and having apolymeric outer portion having a first elastic modulus and a polymericinner portion having a second elastic modulus, wherein the first elasticmodulus differs substantially from the second elastic modulus.
 13. Theimplant of claim 12 wherein the polymeric outer portion has a lowelastic modulus.
 14. The implant of claim 13 wherein the polymeric outerportion comprises at least one polymer selected from hydratable polymerswhich swell upon hydration and hydratable polymers which do not swellupon hydration.
 15. The implant of claim 13 wherein the polymeric outerportion comprises an elastomer.
 16. The implant of claim 13 wherein theouter portion comprises at least one polymer selected from latex rubber,polysilicones, polyurethanes, polyesters, and polyacrylates.
 17. Theimplant of claim 13 wherein the polymeric inner portion has a highelastic modulus.
 18. The implant of claim 12 wherein the polymeric innerportion has a high elastic modulus.
 19. The implant of claim 18 whereinthe inner portion comprises at least one polymer selected from PMMA,poly(tetrafluoroethylene), silicone, polycarbonate, a polyolefinselected from polyethylene, polypropylene, polybutylene, and mixtures orinterpolymers thereof.
 20. An intracorneal implant shaped to be insertedin an intracorneal channel in a circumferential direction about theoptic axis of a human eye and configured to alter the curvature of thecornea to effect refractive correction of an eye, said implantcomprising an elongated arcuate member having an outer surface andhaving ends, wherein the outer surface has a first polymeric portionhaving a first elastic modulus and a second polymeric portion having asecond elastic modulus, and wherein the first elastic modulus differssubstantially from the second elastic modulus.
 21. The implant of claim20 wherein the first polymeric portion has a low elastic modulus. 22.The implant of claim 21 wherein the first polymeric portion comprises atleast one polymer selected from latex rubber, polysilicones,polyurethanes, polyesters, and polyacrylates.
 23. The implant of claim21 wherein the second polymeric portion has a high elastic modulus. 24.The implant of claim 23 wherein the second polymeric portion comprisesat least one polymer selected from PMMA, poly(tetrafluoroethylene),silicone, polycarbonate, a polyolefin selected from polyethylene,polypropylene, polybutylene, and mixtures or interpolymers thereof. 25.An intrastromal implant shaped to be inserted in an intracorneal channelin a circumferential direction about the optic axis of a human eye andconfigured to alter the shape of the cornea by substantially apredetermined amount to effect a desired refractive correction of aneye, said implant comprising an elongated arcuate member having endswhich comprises an inner physiologically-compatible polymeric portionhaving a high elastic modulus and an outer physiologically-compatiblepolymeric portion having a low elastic modulus.
 26. The implant of claim25 wherein the inner portion comprises a single polymeric layer.
 27. Theimplant of claim 26 wherein the outer portion comprises a singlepolymeric layer.
 28. An intracorneal implant shaped to be inserted in anintracorneal channel in a circumferential direction about the optic axisof a human eye and configured to alter the curvature of the cornea toeffect refractive correction of an eye, said implant comprising anelongated arcuate member that comprises a physiologically compatiblepolymeric inner portion having a first elastic modulus and aphysiologically compatible polymeric outer portion having a secondelastic modulus, said outer portion being hydratable and said firstelastic modulus differing substantially from said second elasticmodulus.
 29. The implant of claim 28 wherein the outer portion has a lowelastic modulus.
 30. The implant of claim 29 wherein the inner portionhas a high elastic modulus.
 31. The implant of claim 28 wherein theimplant comprises a split ring.
 32. The implant of claim 28 wherein theouter portion comprises an outer layer comprised of a low elasticmodulus polymer.
 33. The implant of claim 32 wherein the inner portioncomprises an inner layer comprised of a high elastic modulus polymer.